Many cameras are equipped with a light metering system, such as TTL (“through-the-lens”) metering, by which the camera measures light levels in a photographic environment, and/or related to a photographic subject in the environment. In TTL metering, the camera measures light through its taking lens, such as by means of an internal light meter.
Such light metering systems may be used to determine appropriate flash settings for use during image acquisition, a process sometimes referred to as “flash metering.” In TTL flash metering, prior to image acquisition, one or more pre-flash emissions (or “pre-flashes”) are requested by the camera, typically by means of electronic signals sent by the camera to a flash device. In response, the flash device emits a known quantity of light. Light from the pre-flash emission, along with light from other sources in the photographic environment, may bounce off the photographic subject and pass through the camera's taking lens en route to a light meter (for example, a defined field of imaging sensors). Based on the intensity of light detected, the camera determines whether the intensity of the flash emission for image acquisition (sometimes called the “main flash”) should be less than, the same as, or greater than the intensity of light used in the pre-flash emission. Generally, the timing of TTL flash metering is such that the pre-flash precedes the main flash by 10-150 milliseconds, but could be as short as 100 microseconds.
The above explanation of TTL flash metering is somewhat simplified, as various techniques have been developed, especially in digital cameras, such as different ways in which a pre-flash is used, how incident light is measured, and/or the manner in which a flash intensity value may be determined. For example, Canon has developed TTL flash metering systems such as A-TTL (“advanced through-the-lens”), E-TTL (“evaluative through-the-lens,” as described in Canon's U.S. Pat. No. 6,404,987, the complete disclosure of which is hereby incorporated by reference), and E-TTL II, whereas Nikon has developed D-TTL, iTTL, and so forth.
Regardless of the exact technique or protocol employed, the TTL flash metering process is automatic and does not provide for or allow user adjustment of the flash intensity value that is calculated by the camera, or even inform the user of the calculated flash intensity value. Rather, once the flash intensity value is calculated, it is usually almost immediately signaled to the flash device, and then discarded once image acquisition has occurred (e.g. a photograph is taken). The automatic, rapid nature of the TTL flash metering process allows it to be repeated anew every time image acquisition occurs, and is thought to better ensure that the amount of light illuminating the photographic subject is correct despite even minute changes in the photographic environment from one shot to the next.
Some cameras enabled with TTL flash metering also have a flash exposure lock (“FEL”) feature that may allow such a flash intensity value (i.e. one calculated by the camera based on exposure data gathered from a pre-flash) to be retained for a short period of time, such as 15-20 seconds, to allow a user to prepare for and then take several shots using the same flash intensity. However, FEL does not allow user adjustment of flash settings, and the calculated flash intensity value is discarded after the short period of time elapses once shooting is complete. Another somewhat related feature implemented in some cameras is known as flash exposure compensation (“FEC”), which may allow a user to instruct the camera to increment the amount of exposure compensation that a camera makes to an image acquired thereby, but FEC does not allow a user adjust the calculated flash intensity value used during image acquisition. Neither FEL nor FEC, nor any TTL flash metering system, provide feedback to the user as to the flash intensity value that is calculated by the camera for use during image acquisition. Indeed, because the pre-flash emitted by a flash device may be so close in time to the main flash emission so as to be indistinguishable from it, a user may have no indication that a flash intensity value is even calculated.
Rather, user control of various flash settings (including, in some cases, flash intensity) is typically achieved, if at all, via controls on more sophisticated flash devices. Generally, when used in conjunction with a camera, such as by being directly connected to the camera or set up as a remote flash to be controlled by either the camera or a flash device connected thereto, such a flash device may be switched between an automatic (e.g. TTL) mode in which it is slaved to another device (typically, the camera) and a manual flash mode in which a user may set certain flash settings for the flash device.
However, manually setting flash settings on a flash device does not allow a user to leverage the camera's TTL flash metering system, and instead requires the user to rely on guesswork and/or experience in order to predict an appropriate or desired flash intensity for image acquisition. This challenge is further complicated in situations in which more than one flash device is used to provide illumination of a photographic subject, and/or if a user would like to replicate lighting conditions used for an earlier photograph (such as one taken an hour before, or a day, or even longer).